1. Field of the Invention
The present invention relates to an image pickup device for picking up an object to be picked up at a video rate and outputting an image signal, and particularly to an image pickup device for simultaneously and rapidly picking up images of low resolution of the entire object to be picked up and images of high resolution of parts of the object to be picked up.
2. Description of the Related Arts
Among conventional methods for simultaneously picking up a wide range and detailed images, there is one method employing a plurality of cameras of different zoom ratios and another method employing a camera of high resolution. In the method employing a plurality of cameras of different zoom ratios, two cameras, namely a wide-angle camera 100 and a telephoto camera 102 as illustrated in FIG. 1 are employed such that the wide-angle camera 100 acquires images of the entire objective region while for picking up detailed images, an electric universal head mounted to the telephoto camera 102 is controlled so that the telephoto camera 102 is turned to a direction of a required portion. In this manner, it is possible to obtain an overall image and desired partial telephoto (enlarged) images. In the method employing a camera of high resolution, a camera of remarkably high resolution is employed for the pickup and not only required spots but also the entire object is picked up at high resolution. There are known two methods in conventional approaches for obtaining images of high resolution, one of which is a method for realizing high resolution in that the pickup device includes elements that are minute themselves for increasing the number of elements as a whole, while another one is a piece-wise image compositing method in which images that have been picked up piece-wise by using a plurality of pickup devices are composite into a single large image. In the method of employing minute elements for the pickup device, developments have been accelerated in the field of both, CCD pickup devices as well as CMOS pickup devices in view of the spread of digital cameras in these years, and accompanying remarkable increases in the number of elements, cameras of 3 million pixels and more for consumer and industrial use are already commercially available. As for the piece-wise image compositing method, explanations will be made by referring to the pickup device as suggested by Tanabe et al. (Japanese Patent Unexamined Publication No. 9-289605 (1997)). A light ray that has been condensed by a lens of a camera is N branched by means of a branching means that is provided at a rear stage of the lens, and pickup devices such as CCDs are respectively provided for the respectively branched light rays for pickup. At this time, the respective pickup devices are disposed to receive light of mutually different parts of the original image. A group of partial images of the object can be obtained by picking up with the thus disposed devices. The obtained partial images are thereafter composited to obtain a single large image of high resolution. If branched into four, it will finally be possible to obtain an image having a high resolution equivalent to an image that has been picked up by an pickup device having a number of elements that is as four times as large as the original number.
However, the following drawbacks are presented in such a conventional device for simultaneously picking up an overall image and partial images of an object. First, in the method employing two cameras, namely the wide-angle camera 100 and the telephoto camera 102 as illustrated in FIG. 1, the provision of two cameras 100, 102 located at different positions may lead to a drawback that when controlling the telephoto camera 102 to turn to a required spot, the direction may be different depending on respective distance to object bodies 104, 106. That is, even though the object bodies 104, 106 might be located at different positions as illustrated in FIG. 1, the directions (positions) when seen from the wide-angle camera 100 side will be considered to be identical. However, the directions to which the telephoto camera 102 should turn differ depending on the distances (positions) of the object bodies 104, 106. In this manner, even though the directions to which the telescopic camera 102 should turn differ depending on the positions of the object bodies 104, 106, the distance with respect to the object bodies 104, 106 cannot be acquired by the wide-angle camera 100 alone, and it may happen that the direction to which the telephoto camera 102 should be turned is not available. This drawback is one a drawback that occurs owing to the fact that the provision of a plurality of cameras of different positions of viewpoints will inevitably result in a plurality of optical axes.
Next, drawbacks caused in the method employing a camera of high resolution will be explained. First, since the entire image will be of high resolution, an amount of information included in the image will be enormous and leads to a drawback in that image cannot be rapidly transferred. For instance, in case an image of 3 million pixels that has been obtained upon high densification of the pickup device elements is to be transferred, the amount of information will be eleven times as large as an ordinary TV image of NTSC type, and a transfer rate of NTSC signals will be 2.7 frames/second (fPS). When considering an image processing system using a personal computer as a processing device, the transfer rate of a PCI bus that is generally used in a personal computer is 133 MB/second at maximum and the rate at which an image of 3 million pixels can be taken into the computer on the provision of 1 bite/pixel, the transfer rate will be 2.3 frames/second and thus also low. As explained, it is difficult to acquire images at a video rate (30 frames/second) in the method for achieving high resolution of the entire range of pickup owing to shortage of speed for transferring images, and such a method cannot be applied to a moving image processing system that focuses on moving objects.
A second drawback is it that in case a further increase in resolution is required, it will be difficult to achieve high resolution of a desired level. It is generally the case that the resolutions required for image processing differ depending on objects to be picked up, and it may be that some pickups need to be made at extremely high resolution. However, in the method employing a pickup device of high density, the upper limit for improving the resolution will be limited by the resolution owned by the device. While it is necessary to manufacture pickup elements of even higher density for improving the resolution, minute elements result in a lesser amount of photoreceptors and thus in worsening of S/N ratio and is thus not practical. It is also a drawback in view of practicality that micromachining leads to extreme increases in costs owing to difficulties in production. It is similarly difficult to improve resolution on demand in the piece-wise image compositing method in which an image is composited of piece-wise picked up images using a plurality of pickup elements. This is due to the fact that for improving the resolution, it will be required to increase the number of dividing a single image, that is, the number of pickup devices to be used. However, in case the number of pickup devices becomes very large, it will in fact be impossible to secure spaces for disposing the pickup devices within the camera. For instance, for improving the resolution at an aspect ratio of ten times with respect to the overall image, it will be necessary to dispose 100 pickup devices which can hardly be realized.